Technical Field
The present disclosure relates to a supercapacitor with movable separator and to a method of operating a supercapacitor. The present disclosure also relates to a method of manufacturing a package for a supercapacitor, and a supercapacitor thus obtained.
Description of the Related Art
As is known, supercapacitors are devices for energy storage and are encountering an ever-increasing use in different sectors on account of their peculiar features. Supercapacitors are distinguished, in fact, both from conventional capacitors, owing to the higher density of energy stored (J/kg) and from batteries, owing to the higher power density (W/kg). As compared to batteries, in other words, supercapacitors are able to store and supply the energy stored in times that are much shorter, even though the energy totally available is lower.
The fields of use of supercapacitors are extremely varied, both for low-voltage applications and for power applications.
By way of example, at low voltage, supercapacitors are frequently used: as backup source for memory functions in a wide range of devices, such as cellphones, tablets and portable computers; in applications in pulse-width modulation in portable devices that use electromechanical actuators (such as zoom systems and systems for automatic focusing in photographic cameras and video cameras, or devices for parking the read/write heads in many mass-memory devices), in order to prolong the service life of the main batteries; and for storing energy converted by photovoltaic panels. As regards power applications, supercapacitors are advantageously used for example in systems for harvesting the kinetic energy of vehicles by storing energy during braking and returning it during acceleration. Supercapacitors are also used in uninterruptible power supplies (UPSs) for short-period interventions in which a fast action is desired. Combination with supercapacitors also has beneficial effects on the life of the batteries, which thus basically intervene during prolonged interruptions and are generally called on to supply lower peak currents. For this reason, further, smaller batteries may be used.
Supercapacitors generally comprise two electrodes, for example of aluminum or ruthenium oxide, an electrolyte and a separator. The electrodes are arranged at ends of a chamber filled by the electrolyte. The separator, which is defined by a porous diaphragm permeable to the passage of ions, is arranged between the electrodes and prevents short circuits between the electrodes themselves.
The separator is an important element of supercapacitors, because it concurs in determining the equivalent series resistance (ESR) and the power that may be supplied by the supercapacitor, which is the greater the lower the equivalent series resistance. With current manufacturing techniques, the thickness that may be reached for the separators is in the region of 20 μm, a size that fixes the lower limit of the equivalent series resistance and consequently the maximum power that may be supplied.
Another important element of supercapacitors, which is also affected by the separator, is the discharge current. When the power source that determines charging of the supercapacitor is removed, a discharge current is in fact triggered because the porous separator enables a process of reverse migration of the ions that tends to cause the voltage across the electrodes to vanish. The discharge currents are in general significant and determine discharge in relatively short times or require periodic recharging procedures.
Current techniques for filling the chamber with electrolyte are not optimal. In particular, known techniques, which envisage opening of through holes for fluidic access to the chamber (used for introducing the electrolyte), and subsequent closing of said holes, are carried out according to known steps of micromachining of semiconductor materials (e.g., photolithography and etching), with the consequence that the inner chamber may be contaminated by particles deriving from the micromachining process.
Further, the step of closing of the holes for fluidic access to the chamber is, according to the known art, inconvenient. In the first place, it may be noted that classic techniques of “moulding” at high temperature (higher than 150° C.) may not be applied in the presence of electrolytes having a relatively low boiling point (around 100° C.).
Further, it may be noted that it is further inconvenient to dispense sealant material in the case where the holes for access to the inner chamber are made in the area of both of the electrodes (i.e., on both sides of the inner chamber). In fact, in this case, to prevent exit of the electrolyte from one of the holes, it would be necessary to dispense the sealant material simultaneously in both of the holes. This would require the use of systems specifically provided for the purpose, with a further economic burden.
Finally, there are evident the difficulties in handling the wafer in the step that follows introduction of the electrolyte into the inner chamber and that precedes closing of the holes in order to minimize undesirable leakage of electrolyte.